1. Field of the Invention
The present invention relates to a hydraulic control system for an excavator having a swing-independent hydraulic circuit. More particularly, the present invention relates to a hydraulic control system for an excavator provided with an improved swing-independent hydraulic circuit, which can independently control a swing motor, and efficiently utilize the hydraulic capability of a swing drive system by making the hydraulic fluid being supplied from a swing hydraulic pump join the hydraulic fluid in working devices when the working devices, such as a boom, an arm, and the like, are compositely driven.
2. Description of the Prior Art
In heavy construction equipment, such as an excavator, a loader, and the like, diverse attempts to efficiently control the horsepower or fluid pressure of an engine have been made, and in the case of compositely operating a swing structure and a working device, such as a boom, an arm, or a bucket, it is required to efficiently control not only the engine but also the hydraulic system.
A typical hydraulic control system for an excavator having a confluence circuit for connecting a hydraulic pump, a traveling device, and working devices has been disclosed. In order to heighten the operation speed and the manipulation of the respective working devices, the confluence circuit makes the hydraulic fluid in the hydraulic pump connected to the traveling device join the hydraulic fluid in the working devices, and thus the hydraulic circuit becomes complicated.
FIG. 1 is a view schematically illustrating a conventional excavator that is heavy construction equipment, and FIG. 2 is a view schematically illustrating the construction of a hydraulic system for the excavator as illustrated in FIG. 1.
According to the excavator as illustrated in FIG. 1, an upper swing structure 1 is mounted on an upper part of a lower driving structure 2, and on the upper swing structure 1, a cab 3 installed in front of an engine room 4, and working devices including a boom 5, an arm 7, and a bucket 7, are mounted.
Typically, in the engine room 4, an engine, a radiator, a radiator fan, an oil cooler, and an oil cooler fan are installed, and a main pump and a small pump for operating the oil cooler fan and the radiator fan pump the hydraulic fluid from a hydraulic tank T through the rotation of the engine. Also, plural actuators including a boom cylinder 9, an arm cylinder 11, a bucket cylinder 13, a swing motor, and so on, are driven by the fluid pressure of the hydraulic fluid discharged from hydraulic pumps 201 and 206.
Referring to FIG. 2, the first hydraulic pump 201 supplies the hydraulic fluid to a first traveling control valve 202, a first boom control valve 203, a first swing control valve 204, and a first arm control valve 205.
Also, the second hydraulic pump 206 supplies the hydraulic fluid to a second traveling control valve 207, a second boom control valve 208, a second bucket control valve 209, and a second arm control valve 210. Accordingly, the first traveling control valve 202 controls a left traveling motor 211 in accordance with the fluid pressure applied from the first hydraulic pump 201, and the second traveling control valve 207 controls a right traveling motor 212 in accordance with the fluid pressure applied from the second hydraulic pump 206. The bucket cylinder 13 is controlled by the second bucket control valve 209, the boom cylinder 9 is controlled by the respective boom control valves 203 and 208, and the arm cylinder 12 is controlled by the respective arm control valves 205 and 208.
In the parallel hydraulic circuits using two hydraulic pumps as described above, the hydraulic fluid flows to a side where the resistance caused by the fluid pressure is high, and thus a relatively low fluid pressure appears in a circuit having a high resistance. Accordingly, in the case of compositely operating the swing motor and the arm, or the swing motor and the boom, the actuator may not operate smoothly to lower the driving speed of the actuator.
Particularly, if an actuator for another working device is driven while the fluid pressure is required for the swing operation, the fluid pressure being applied to the swing motor is decreased to lower the original swing speed. Accordingly, in order to perform an efficient composite operation, a swing-independent hydraulic control system, in which the fluid pressure is provided through a separate hydraulic pump, is required so that the swing motor is not affected by other actuators.
However, as illustrated in FIG. 3, the conventional swing-independent hydraulic control system has the drawback that, although the performance of swing composite operations is improved through the independent control of the swing motor 204, it is inefficient in controlling the flow rate or the horsepower of the engine. That is, since the swing motor 204 is not used in the case of performing the digging operation, the third hydraulic pump 213 is in an idle state, and this causes the performance of the flow rate control to be lowered.
In addition, although the performance can be maintained in the case where the boom, the arm, and the like, are compositely operated by the first and second hydraulic pumps, respectively, it is impossible to use the fluid pressure of the third hydraulic pump required for the actuator in the case where the swing motor and the boom, or the swing motor and the arm are compositely operated.